A pronounced micrencephaly, characterized by a great reduction in the neocortex and limbic cortex of the rat brain, is induced by injecting pregnant females with methylazoxymethanol acetate (MAM) on the 15th day of gestation. The objective of this study was to determine if micrencephalic rats were capable of successfully inhabiting large, complexly structured environments while living as free-ranging social groups. As in previous studies it was demonstrated here that MAM-induced micrencephalics are capable of successful reproduction in restricted, laboratory cage environments. However, prior to this study it was not known if the entire mantle of cerebral cortex was required to enable a mammal such as the rat to cope successfully with the dynamic situations known to occur in free-ranging, quasi-natural, laboratory populations. Our results show that micrencephalic rats do not adjust a new social/environmental conditions as quickly as controls: Upon introduction to a new situation micrencephalic rats are much more aggressive and have about three times as many wounds as controls. Also, initially micrencephalic animals have less reproductive success than that of controls; in first litters micrencephalic females rear to the weaning age only 13 percent of the number of pups nurtured by control females. After a prolonged period of adjustment, micrencephalic females are about 50 percent as successful as control females in rearing pups to the age of weaning. Both micrencephalics and controls are influenced by the stress resulting from increased population density, with the effect being more pronounced in the micrencephalics. At a population density of 16 adults per environment, control rats are only about 56 percent as successful, and micrencephalics about 28 percent as successful, as are equivalent rats housed at a density of eight adults per environment. Observations of other behaviors also indicate that the treated animals have difficulties in coping with social and environmental complexity especially upon initial exposure to new situations. A major conclusion based on results of this study is that adult rats handicapped by arrested neocortical development may require a longer period of adjustment to new social and environmental situations than experienced controls.